As they age the contacts of such control switches display an increased contact resistance, which moreover depends on the position of the control switch. This increase is caused by the deposition of carbon on the contacts during decomposition of the oil present in the switch. When the contact resistance increases the contacts burn in, which can result in breaking of the metallic contact. The then occurring electric arcs lead to burning away of the contacts, and a malfunction is then a fact.
These control switches are generally arranged in the case of a transformer, so that, in order to gain access to such a control switch, the oil must be drained from the transformer case and the control switch removed from the case. This is an expensive operation which must preferably be performed only when it is really necessary to recondition the control switch or perform other operations on the control switch.
There thus exists a need for a method for measuring the contact resistance of such a control switch without having to gain physical access to the control switch.
Put another way, there exists a need for a method for measuring a resistance value in a network incorporated in such an electrical apparatus.
It is of course possible to measure the DC resistance of a control switch and, connected thereto in series, the ohmic resistance value of a winding using a Thomson bridge, or by making use of a four-point measurement.
The fact that the winding is connected in series to the resistance whose value has to be measured has the result herein that, as a consequence of the self-induction of the winding, long wait times are necessary before the current becomes stationary. In typical grid transformers these wait times lie in the order of magnitude of about ten minutes.
The measuring of the contact resistance of a control switch in all positions and all three phases of the transformer is thus an activity taking up a particularly large amount of time.
The above stated known method is otherwise only suitable for measuring the stationary resistance of a control switch in a particular position of this switch. However, there is also often a need for information concerning the resistance of the switch during switching. At the moment an additional measurement is necessary for this purpose.
A known method of measuring the resistance change during switching makes use of an oscilloscope. The high-voltage winding of a phase is herein connected to a direct voltage source and the associated low-voltage winding is connected to the oscilloscope. Herein is assumed a situation in which the control switch is accommodated in the high-voltage winding. A change in the resistance on the primary (high-voltage) side during switching will result in a voltage change on the secondary side, which is visible on an oscilloscope. The value of the diverse contact resistances in the control switch can with some calculation be derived from the thus obtained oscillograms. This always relates however to dynamic effects; this measurement does not provide a definite answer concerning stationary contact resistances.
Another method of drawing some conclusion about the resistance change during switching makes use of a measuring bridge to determine the transformer ratio of a transformer. Loss of balance of the measuring bridge, which is visible as a movement of the needle of the null indicator during switching of the control switch, indicates a change in the contact resistance. The magnitude of the deflection of the null indicator gives some qualitative indication concerning the contact resistance but does not automatically result in quantitative values for the contact resistance.